In many contexts it is desirable to detect electromagnetic radiation signals, such as in spectroscopy, refractive index measurements or the like. Conventionally, such radiation may be detected by means of a power detector whose output voltage is proportional to input power (watts). Such power detectors include, for example, thermistor bolometers, thermopile detectors, silicon diode photon detectors and the like. In an alternative technique, an electric-field sensing system detects electromagnetic radiation by producing an output voltage proportional to the electric-field strength (volts/meter) in the region around an input antenna. Examples of these systems include, for example, conventional radio receivers, television receivers, radar receivers and the like. Electric-field sensing systems lend themselves to heterodyne detection and typically exhibit a significantly lower noise-equivalent power than power detectors, and are therefore better adapted to receive lower-power beams. Nonetheless, in various contexts, the maximum sensitivities permitted by such conventional electric-field sensing systems are being challenged, and even exceeded.
More particularly in the context of spectroscopy, for example, data for chemical and biological agents such as the Anthrax stimulant Bacillus Subtilis (BG) indicates the need for spectral measurements over a wide frequency range with challenging sensitivity. Typically, measurements are taken from dense samples of BG spores prepared at concentrations high enough to be measured by a conventional Fourier Transform spectroscopy (FTS) instrument. At these concentrations, resonances may be damped by the density of spores in the sample. As will be appreciated, measurements on samples in low density aerosol form may provide better characterization of biological warfare agents (BWAs) in representative scenarios. However, measurements on such samples require spectrometer sensitivities not available in today's laboratory spectrometers. As such, it would be desirable to design a system and method for detecting very low power electromagnetic signals absorbed by low concentration samples with better sensitivity than that of currently available systems and methods.